Patients with acute respiratory failure in need for ventilator treatment in intensive care units show highly varying pathophysiologic conditions of the respiratory system. With regard to the heterogeneity of acute lung injury (ALI) and the more severe acute respiratory distress syndrome (ARDS), the percentage of potentially recruitable lung, i.e. lung tissue that was collapsed but can be opened by a high pressure inflation is up to approximately 60%. One important reason for the heterogeneity is whether the patient has ARDS of pulmonary or extrapulmonary origin, i.e. whether it is the lungs per se or the chest wall and the diaphragm that are mainly affected. In most cases of respiratory failure, both the mechanical conditions of the lung, the stiffness (elastance=E) of the lungs (El) and the stiffness of the containing wall, (Ec), chest wall and diaphragm, play an important role.
During ventilator treatment the mechanical properties of the total respiratory system was hitherto determined by the combined effect of stiffness of the lungs and the stiffness of the chest wall/diaphragm working in series. The lung is a compliant unit within another compliant unit, namely the chest wall and the diaphragm. For optimal ventilator treatment, where risks and benefits of the treatment are balanced, knowledge of the stiffness of the chest wall in relation to the stiffness of the lung is of outmost importance.
For instance, the risk of inducing damage to the sensitive lung tissue by the ventilator treatment is increasing when the lung is very stiff and the chest wall/diaphragm is very soft, where most of the airway pressure generated by the ventilator during inspiration acts solely on the lung, i.e. a high transpulmonary pressure is present. Very little of the pressure applied by the ventilator to the patient is transmitted to the surrounding chest wall and diaphragm.
On the other hand, in a case where the stiffness of the chest wall and especially the diaphragm is increased, e.g. by abdominal inflammation, with resulting high sub-diaphragmatic pressures, the lung will be limited in its expansion by the stiffness of the chest wall and diaphragm, and the transpulmonary pressure will be low. The risk of ventilator induced lung injury will be reduced. This was shown by Talmor et al, in a randomised study on oesophageal pressure guided mechanical ventilation in ARDS patients (Talmor et al NEJM 2008; 359(20): 2095-2104).
As measurement of the transpulmonary pressure is difficult, a measurement of the oesophageal pressure is used as a surrogate of pleural pressure instead of measuring pleural pressure directly. The oesophageal pressure is used as an indirect measure of how much airway pressure is transferred through the lung to the chest wall and diaphragm during assisted controlled ventilation. This makes it possible to give an estimate of the stiffness of the chest wall/diaphragm based on the oesophageal pressure.
The combined stiffness of the lungs and chest wall/diaphragm, total respiratory system stiffness (Etot), is the sum of the lung stiffness and the chest wall/diaphragm stiffness. The stiffness of the lung may thus indirectly determined by subtraction of Ec from Etot. The calculation of chest wall and lung compliance is based on the tidal difference in end-expiratory and end inspiratory oesophageal and airway pressures (ΔPoes, ΔPaw).
However, there are practical difficulties of performing the oesophageal pressure measurement. Oesophageal pressure is measured by means of catheter like elongate pressure monitoring devices, such as disclosed in U.S. Pat. No. 4,214,593. The device comprises a nasogastric tube provided with an oesophageal balloon cuff.
The correct placement of the oesophageal balloon catheter in the oesophagus, especially in patients who already have a stomach tube inserted through the oesophagus, has shown to be very difficult. It can be compared with forwarding a soft spaghetti through a branched tubing structure without vision during this action.
Moreover, the performance of the oesophageal balloon as a transmitter of oesophageal pressure is influenced by how much it is preinflated and how much mediastinal weight, i.e. weight of the heart is superimposed on the balloon. Also, the reliability of the measurements has been questioned as oesophageal pressure is a substitute measure of pleural pressures, which are different in different places, due to gravitational forces and its proximity to the diaphragm, where abdominal pressure and diaphragmatic stiffness have a greater impact.
In addition, an oesophagal balloon measurement provides a pressure measurement only for the horizontal plane in which the measurement is done. Depending on the positioning in the patient thus different measurement values will be obtained e.g. due to gravitational forces acting on the patient body and in particular the lung, directly or indirectly via the weight if other organs in the thorax of the patient. There is a need of providing a measure representative of all transpulmonary pressures irrespective of the position thereof, avoiding the influence of factors such as gravitational forces acting on the patient body.
Thus, besides the costs for the catheters and their use, the practical positioning difficulties and doubtful reliability of measurement values obtained have resulted in a very limited clinical use of such oesophageal balloon catheter.
Another important issue is that measuring pleural pressure directly in the pleural cavity surrounding the lungs is practically not possible as the pleural space usually is very small and a risk of puncturing the lungs is impending but should be avoided by any means. It is highly hazardous to measure the pleural pressure due to the risk of puncturing the lung. Therefore, it has been attempted to use oesophagal pressure as a surrogate as described above.
Hence, there is a need for a new or alternative way of measuring or determining transpulmonary pressure in a patient connected to a breathing apparatus.
European Patent publication EP1295620 discloses a breathing apparatus for use in the examination of pulmonary mechanics of a respiratory system.
TALMOR DANIEL ET AL: “Mechanical Ventilation Guided by Esophageal Pressure in Acute Lung Injury”, NEW ENGLAND JOURNAL OF MEDICINE vol. 359, no. 20, November 2008 (2008-11), pages 2095-2104, discloses a study concerning randomly assigned patients with acute lung injury or ARDS to undergo mechanical ventilation with PEEP adjusted according to measurements of esophageal pressure (the esophageal-pressure—guided group) or according to the Acute Respiratory Distress Syndrome Network standard-of-care recommendations.
International PCT publication 2007/082384 discloses a method for determining dynamically respiratory feature in spontaneously breathing patients receiving mechanical ventilatory assist.
An object of the present invention may be regarded as direct determination of transpulmonary pressure without the use of oesophageal pressure measurements.
An improved or alternative system, computer program and/or method for determination of transpulmonary pressure without the use of indirect measures, such as oesophageal pressure measurements would be advantageous. Moreover, it would be advantageous and to provide such a system, computer program and/or method allowing for increased flexibility when using existing breathing apparatuses, cost-effectiveness by avoiding purchase and use of additional equipment needed for the transpulmonary pressure determination, and user friendliness thereof would be advantageous. It would also be advantageous if such a measurement or determination provided a mean value for the transpulmonary pressure, i.e. a measure representative of all transpulmonary pressures irrespective of the position thereof, e.g. due to gravitational forces acting on the patient body.